1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray source, an X-ray detector, and an X-ray filter which is arranged between the X-ray source and the X-ray detector, which X-ray filter includes a plurality of filter elements having an X-ray absorptivity which can be adjusted by controlling a quantity of X-ray absorbing liquid within the individual filter elements, where individual filter elements communicate with the X-ray absorbing liquid by way of a first end.
2. Description of Related Art
An X-ray examination apparatus of this kind is known from French patent application FR 2,599,886.
The known X-ray examination apparatus comprises an X-ray filter for limiting the dynamic range of an X-ray image, being the interval between the extremes of the brightness values. An X-ray image is formed on the X-ray detector by arranging an object, for example a patient to be examined, between the X-ray source and the X-ray detector and by irradiating said object by means of X-rays emitted by the X-ray source. If no precautions are taken, a large dynamic range of the X-ray image may occur. On the one hand, in some parts of the object, for example lung tissue, the X-ray transmissivity will be high whereas other parts of the object, for example bone tissue, can hardly be penetrated by X-rays. If no further precautions are taken, therefore, an X-ray image with a large dynamic range is obtained whereas, for example medically relevant information in the X-ray image is contained in brightness variations in a much smaller dynamic range; because it is not very well possible to make small details of low contrast suitably visible in a rendition of such an X-ray image, such an X-ray image is not very well suited for making a diagnosis. When the X-ray image is converted, using an image intensifier pick-up chain, into a light image which is picked up by means of a video camera, the dynamic range of the light image may be much greater than the range of brightness values that can be handled by the video camera without causing disturbances in the electronic image signal.
In order to limit the dynamic range of the X-ray image, the known X-ray examination apparatus includes an X-ray filter with filter elements provided with a bundle of parallel capillary tubes, each of which is connected, via a valve, to a reservoir containing an X-ray absorbing liquid which suitably wets the inner walls of the capillary tubes. In order to fill one of the capillary tubes with the X-ray absorbing liquid, the valve of the relevant capillary tube is opened after which the capillary tube is filled with the X-ray absorbing liquid by the capillary effect. Such a filled capillary tube has a high X-ray absorptivity for X-rays passing through such a filled capillary tube in a direction approximately parallel to its longitudinal direction. The valves are controlled so as to ensure that the amount of X-ray absorbing liquid in the capillary tubes is adjusted in such a manner that in parts of the X-ray beam which pass through parts of low absorptivity of the object filter elements are adjusted to a high X-ray absorptivity and that filter elements in parts of the X-ray beam which pass through parts of high absorptivity of the object, or are intercepted by a lead shutter, are adjusted to a low X-ray absorptivity.
In order to change the adjustment of the filter of the known X-ray examination apparatus it is first necessary to empty filled capillary tubes. Therefore, use is made of a paramagnetic X-ray absorbing liquid which is forced out of the capillary tubes by application of a magnetic field. After all capillary tubes have been emptied, the X-ray filter is adjusted anew by deactivation of the magnetic field and by subsequently opening valves of capillary tubes which are to be filled with the X-ray absorbing liquid for the new filter adjustment so as to adjust these tubes to a high X-ray absorptivity. Consequently, it is not very well possible to change the adjustment of the known X-ray filter within a brief period of time, for example one second. Therefore, the known X-ray apparatus is not suitable for forming successive X-ray images at a high image rate while changing the adjustment of the filter between the formation of successive X-ray images.
In order to control the quantity of X-ray absorbing liquid in the capillary tubes it is necessary that the period of time during which the valves are opened is accurately controlled; however, the mechanical drive of the valves, for example exhibiting inertia and play, impedes fast and accurate control of the quantity of X-ray absorbing liquid in the capillary tubes.